The present invention relates to downhole tools and, in particular, though not exclusively, to a tool which supports the weight of a work string within a well bore, as typically utilised in oil and gas production.
During drilling or completion of a well, a work string comprising connected sections of tubing is typically inserted in the well bore. Each additional section increases the weight exerted on the string. This weight is required for some downhole operations such as drilling where the weight ensures a drill bit, mounted on the end of the work string, is held against a formation.
Additionally weight set tools have been developed which generally operate by landing a part of the tool on a shoulder of the tool or a formation within the well bore. The contact surface then allows the tool to bear the weight of the string, and in bearing the weight of the string, parts within the tool are moved in relation to each other to perform a function required by the tool.
Such a weight set tool, for example, is that in UK Patent No 2272923 which describes a downhole tool for circulating fluid. This tool includes a sleeve which engages a shoulder portion in the well bore. When engagement occurs, the weight of the string bears down on the tool, moving the tool in relation to the sleeve which is now static against the formation. The change in relationship between the tool and the sleeve aligns ports between a bore of the tool and the sleeve to provide for the radial passage of fluid from the bore of the tool.
A disadvantage of such weight set tools are that they can be activated prematurely whenever an apparent weight or, alternatively, contact with a formation in the well bore occurs. These conditions generally prevail when the body of the tool experiences a pressure differential across its surface, causing the body to expand longitudinally. A temperature differential across the body may have the same effect, as can changes in the body dimensions caused by changes resulting from different plugs of fluids passing down the bore of the tool and up the annulus between the string and the well bore wall or tubing wall if inserted.
It is an object of at least one embodiment of the present invention to provide a downhole tool which prevents the premature activation of weight set tools mounted on a string below the device.
It is a further object of at least one embodiment of the present invention to provide a downhole tool which can be selectively actuated to support at least part of the weight of a string to which it is attached.